Neil Sperry: Plants need a shot of iron, too

Posted Friday, Jun. 13, 2014  comments  Print Reprints
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Nitrogen, phosphorus and potassium are the three heavy hitters when it comes to plant nutrition. They’re the elements that are needed by the bagful to keep our plants healthy and vigorous.

Of course, analyses of clay soils like we have here in the blackland prairie have shown that we seldom need any additional phosphorus and probably not much potassium, either. So we’re not surprised when the garden-center operator tells us that a particular all-nitrogen plant food is all that we need to be buying.

But slow down just a little there, partner! There’s another group of nutrients called “minor” elements that also factor into our successes as gardeners.

That term minor element might lead you to think that these are lesser players in our plants’ vigor, but that’s the wrong line of thinking. They’re just as critical to plant growth and flowering as any of the other big three, they’re just needed in tiny amounts.

The cast of characters includes boron, manganese, magnesium, copper, zinc, iron, calcium, sulfur, molybdenum, chlorine and nickel.

And that is where our story gains speed. We’re going to zoom in on one particular nutrient from all of the baker’s dozen. Iron. Iron is important to plants just as it’s important to people. But not everyone knows what its shortages look like or what they can do to correct them.

Deficiency symptoms

Iron is critical to the formation and function of chlorophyll. Its deficiency, often referred to as “iron chlorosis,” is one of the most commonly seen nutrient shortages in North Texas landscapes. It typically first appears with light green or yellow leaves, with the veins remaining dark green the longest. Eventually, even the veins lose their pigmentation as the leaves turn pale yellow, sometimes almost white.

Eventually the leaves begin to brown and turn crisp.

Since iron becomes affixed within leaf tissues, it does not migrate up in the plant as the stems grow. That’s why iron deficiencies are always most pronounced at the ends of the branches. Older leaves (farther down on the stems) remain green the longest.

And, affected leaves remain on the plant without falling. Hang onto that fact. It will become important in just a moment.

Why iron deficiency happens

You may wonder how a nutrient element that’s needed in such small quantities could ever be deficient in soils. The answer takes you back to some of your basic chemistry courses — iron is insoluble in alkaline conditions. Our blackland clays are highly alkaline. Our irrigation water out of area lakes is even more so.

That’s a bad combination if you’re trying to grow plants that require lots of iron. While there may be ample iron already in our local soils, it remains insoluble, so there’s no way for roots to bring it into the plants.

How we can deal with chlorosis

This is where a list of your options may help you.

• Avoid plants that require acidic soils and high levels of soluble iron. That’s especially true for trees and large shrubs. You won’t be able to accommodate their giant appetites, as they grow larger.

• If you want to grow azaleas, camellias, gardenias, wisterias, dogwoods and other small to midsized plants that require high levels of soluble iron, prepare their planting beds carefully.

Excavate 10 to 12 inches down into the ground, and remove the black clay soil entirely. Backfill with 20 to 24 inches of a mix of sphagnum peat moss and finely ground pine bark mulch. The raised bed you’ll end up with should be 5 or 6 feet wide for azaleas, and up to 15 or 20 feet for dogwoods.

The height of the bed will drop as the organic matter decays, until you eventually will need to remove the plants and rework their beds.

• If iron deficiency does show up in your plants, you’ll find myriad products aimed at correcting it. Some aren’t especially effective, however. You can add sulfur as a soil-acidifier. Your local nursery or farm supply store will have suitable options. And you can apply chelated iron as a foliar spray for quickest response, albeit short-term.

Just to have answered the inevitable question, iron filings are virtually useless as a source of nutrient iron. They start out their life in the soil as insoluble iron, and nothing much happens to change that. Save the trouble.

Critical note: All iron products can leave rusty stains on masonry and painted surfaces. Take extreme care to avoid spray drifts where you don’t want the stains, and sweep all granules that might have ended up on the sidewalk, patio or drive.

Iron chlorosis look-alikes

A couple of other plant problems may resemble iron deficiency symptoms. It’s important to now what they are, so you can be sure of your diagnosis.

Low nitrogen levels also cause light green foliage, but the veins change at the same time as the rest of the leaves, and this shortage will be evenly distributed up and down the plant’s stems.

Moisture stress can cause large-leafed plants such as fruitless mulberries to develop yellowed leaves, sometimes even with dark green veins. However, they’ll be older leaves, and they will fall from the trees within just a few days. Iron-deficient leaves remain attached for months.

Neil Sperry publishes Gardens magazine and hosts “Texas Gardening” from 8 to 10 a.m. Sunday on WBAP AM/FM. Reach him during those hours at 800-288-9227. Online: www.neilsperry.com.

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